This invention relates to nuclear reactors, and more particularly to fuel rods for pressurized water reactors.
Fuel rods in a pressurized water reactor are typically stacks of uranium dioxide pellets which are contained in an outer cladding of Zircaloy or other material. These fuel rods are formed into bundles comprising a fuel element. Each fuel element also contains a top and bottom end plate as well as several spacer grids along the axial length of the fuel element to hold the fuel rods in place.
For a number of years, the use of annular fuel pellets rather than the conventional solid fuel pellet has been studied. Such an annular fuel pellet is typified by a cross-sectional configuration in which the central part of the pellet is hollow. The use of annular fuel pellets offers a number of advantages over the use of a conventional solid fuel pellet. Firstly, a pellet with a central void or hollow region reduces the maximum fuel temperature. This can provide extended life of the fuel rods and therefore reduction in cost of materials and time to replace spent fuel rods. Secondly, gaseous fission products accumulate within the fuel rod during the life of the fuel. The use of a central void region within the fuel rod acts as a plenum for these gaseous products.
Along with the advantages of the use of an annular fuel pellet, certain problems also arise. One of the main problems in connection with the use of an annular pellet is the possibility that fragments of the nuclear fuel may break off internally and fall into the central void area. These fragments may fall to the bottom of the respective fuel rod and accumulate there. Alternatively, larger fragments may lodge somewhere along the axial length of the central void region. In either event, these fragments or debris may cause power peaking problems, and they would reach a very high temperature and possibly melt due to poor heat transfer. This fragmentation problem is caused by thermal cycling during operation of the nuclear reactor which can result in erosion of the pellets. An ancillary concern is the creation of uncertainty in estimating system conditions after such fragmentation has occurred.
U.S. Pat. No. 4,273,616 discloses an arrangement in which spacer plugs are located at the fuel assembly grid elevations along the fuel rod. These plugs physically trap or catch pellet debris to avoid excessive power peaking and power distribution uncertainties. However, this arrangement inherently allows for redistribution of fissile material axially within the fuel rod and a reaccumulation of debris along the upper surface of each spacer plug. It also fails to prevent further erosion and fragmentation of the annular fuel pellets.